1. Field of the Invention
The invention relates to N.sup..epsilon. -trifluoroacetyl-lysyl-proline aromatic sulfonic acid salts.
2. Description of the Related Art
The synthetic intermediate peptide lysyl-proline and its derivatives are useful in preparing important products such as Tuftsin which possesses an immunity-increasing action (Tuftsin: L-threonyl-L-lysyl-L-prolyl-L-arginine, Ann. N.Y. Acad. Sci., vol. 419, 12, 1983) and Lisinopril which possesses a blood pressure-lowering action (Lisinopril: N-.alpha.-(1(S)-carboxy-3-phenylpropyl)-L-lysyl-L-proline, Japanese first patent publication No. 81845/1980), and others. Synthetic procedures for producing such compounds are often based on the important dipeptide derivative N.sup..epsilon. -trifluoroacetyl-L-lysyl-L-proline as a starting material. This starting material has been found to be unstable and is easily converted into a diketopiperazine (II) as shown in the following equation: ##STR2## There is no known stable and easily handled form of N.sup..epsilon. -trifluoroacetyl-L-lysyl-L-proline. In view of the importance of this dipeptide derivative as a starting material, and the fact that known forms of this starting material are unstable and hard to manage, there remains a great need for new and more stable forms of this starting material. In particular, there is a need for a stable crystal of N.sup..epsilon. -trifluoroacetyl-L-lysyl-L-proline in the free form or in stable salt form.